1. Field of the Invention
The present invention relates to a pressing apparatus, and more particularly, to a pressing apparatus for de-watering a continuous web, such as a paper web.
2. Description of the Related Art
For many years attempts have been made to use external air pressure to force water out of a paper web. Rather than compress a sheet at a press nip to the point where hydraulic pressure drives water out, as is the case in normal wet pressing, it was reasoned that more water could be removed, and sheet bulk could be maintained, if air pressure could be applied to supplement or replace roller nip generated hydraulic pressures. One such attempt involves providing a multi-roller structure forming a closed chamber, wherein air is circulated from the chamber through the roll surface to convect moisture out of the paper web that is wrapped over the roll.
Also, it has been recognized that conventional wet pressing methods are very inefficient in that only a small portion of a roller""s circumference is used for processing the paper web. To overcome this limitation, some attempts have been made to adapt a solid impermeable band to form an extended nip for pressing the paper web to de-water the paper web. One problem with such an approach, however, is that the impermeable band prevents the flow of a drying fluid, such as air, through the paper web.
Accordingly, a need exists for an improved pressing apparatus which provides enhanced de-watering of a continuous web by simultaneously effecting both a predetermined fluid flow through and a mechanical pressing force on a continuous web.
The present invention provides a pressing apparatus which provides enhanced de-watering of a continuous web by simultaneously effecting both a predetermined fluid flow through and a pressing force on a continuous web.
The invention comprises, in one form thereof, an apparatus for processing a continuous web having a first side and a second side. A plurality of rollers are arranged for cooperative rotation, wherein each of the plurality of rollers has a first circular end, a second circular end and a cylindrical middle surface. The plurality of rollers are positioned to define a corresponding plurality of nips. The continuous web is processed through at least two of the plurality of nips. At least a first roller of the plurality of rollers has at least one void formed in the cylindrical middle surface. First and second sealing panels engage the first and second circular ends of each of the plurality of rollers. The first and second sealing panels and the plurality of rollers define a chamber. A first pressure source is fluidly coupled to the chamber to pressurize the chamber. A membrane is positioned adjacent the first side of the continuous web to separate the continuous web from direct communication with the chamber. The membrane is structured and adapted to have a permeability which permits a predetermined fluid flow therethrough to the continuous web, and structured and adapted for communicating with the pressurized chamber and the at least one void to apply a mechanical pressing force to the continuous web.
In another aspect of the invention, a pressing assembly defines a chamber having an inlet and an outlet. A first pressure source is fluidly coupled to the chamber to pressurize the chamber with a fluid. A membrane is positioned adjacent the first side of the continuous web. The continuous web and the membrane enter the chamber at the inlet and exit the chamber at the outlet. A differential pressure source is coupled to the chamber to effect a flow of the fluid through the membrane and the continuous web, which in turn effects a fluid flow between the chamber and the differential pressure source. The membrane is structured and adapted to have a permeability which permits a predetermined flow of the fluid therethrough to the continuous web, and structured and adapted for communicating with the pressurized chamber and the differential pressure source to apply a mechanical pressing force to the continuous web.
In still another aspect of the invention, first and second sealing panels and a plurality of rollers define a plurality of chambers. At least a first pressure source is fluidly coupled to each of the plurality of chambers to pressurize the plurality of chambers. At least one semipermeable membrane is structured and adapted to engage a portion of a plurality of inlet roller nips, to engage the cylindrical middle surface of a portion of a plurality of main rollers and to engage a portion of the plurality of outlet roller nips to define a plurality of expanded nips.
An advantage of the present invention is that the invention simultaneously effects both a predetermined fluid flow through and a mechanical pressing force on a continuous web, such as a paper web, to promote enhanced de-watering of the continuous web.
Another advantage of the present invention, when multiple chambers are defined, is the ability to simultaneously effect both a predetermined fluid flow through and a mechanical pressing force on a continuous web in a first direction in a first chamber, and simultaneously effect both a predetermined fluid flow through and a mechanical pressing force on a continuous web in a second direction opposite to the first direction in a second chamber to effect de-watering through both major surfaces of the continuous web.